I wouldn't wait much longer. Growth is exponential up to a point that you are far from currently. It only gets faster with time... 4 plants in a 3x3... maybe 21-28 days tops are necessary. shoot for 450g total, that's 112.5g each.. 3-4 weeks still sounds good. YMMV on growth rate, adjust as needed. Heavy yielding genetics will do better ethan 50g/sq ft.
when it fills about 1/2 of the area, you are good to go. 4.5 sq ft give or take.. takes a little imgination.. if they are roughyl 1x1 you are nearly there.
If you flip without overgrowing, you'll eliminate a lot of wasted time and electricity while pruning off less plant over time. If any plant is not growing at a similar rate, may need to compensate a bit or allow a little more vege time etc... do what you have to do.
As far as minimum time to flip? similar to autoflowers 2+ weeks. I'd wager there is genetic variety for any 1 plant just as some plants only need 10 hours of darkness to flip to flower and others need more. I've never done less than 4 weeks, and i've never seen any correlation to better or worse results due to length of vege. At worst it just sits in 12/12 until it is biologically capable of going into flower.